This invention relates generally to a method and apparatus for remotely probing a transmission medium to measure quantities such as velocity and refractivity structure and more particularly to such a method and apparatus which propagates a spatially weighted wave through the transmission medium and spatially weights the received wave to generate an electrical signal indicative of the measured quantities at a particular location in the transmission path of the wave.
Only in recent years has interest arisen in the use of line-of-sight wave propagation as a probe of velocity and turbulence in the atmosphere. Probably, this is due to the fact that only in that period has there been sufficient theoretical understanding of propagation in random media.
Radar in its various forms has, of course, long been used for probing atmospheric conditions. Radar, however, is distinguished by the use of pulses or other modulation to achieve distance resolution. The present invention is concerned with a simple, usually continuous-wave transmitter and a separated receiver. As a consequence, other techniques are required to achieve resolution in space.
The theory of weak scattering in media with an irregular refractive index ("star twinkling") was well developed in the 1960's; see, for example, Proceedings of the IEE, Vol. 57, No. 4, April 1969, p. 375, wherein there appears an article co-authored by myself and Mr. Jeffrey C. Harp entitled "Weak Scattering in Random Media, with Applications to Remote Probing." Simple correlation techniques have been used to measure an "average" velocity and turbulence level over the transmission path. In these techniques, the fluctuations from a single transmitter are observed by two spaced receivers and the time delay between the two receiver outputs noted. Such a technique has been recently incorporated into apparatus which is in commercial production.
There is a large interest presently in discriminating among various portions of the propagation path. One method has been the so-called cross-path correlation technique. In this technique two transmitters are used, with a receiver for each. The paths of the two systems are set up to cross at a point of interest. Thus, any similarities in the output of the two receivers usually arise in the vicinity of the path crossing. In August 1973 at the URSI Convention, I described an atmospheric probing method for measuring refractive irregularities and transverse winds employing two wave sources and spatial and temporal filtering.